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make code more efficient by searching only for the first occurrence rather than all

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edited Sep 24, 2014 at 5:22

Well here's a way. First collectFind the positionsposition of allthe first occurrence of x:

posexpr = Position[HoldForm[xHoldForm[x + 2 + 4 + x]x];

pos = Position[expr, x];x, -1, 1];

Then:

ReplacePart[HoldForm[x + 2 + 4 + x]ReplacePart[expr, First[pos]pos -> 4]

4 + 2 + 4 + x

Well here's a way. First collect the positions of all x:

pos = Position[HoldForm[x + 2 + 4 + x], x];

Then:

ReplacePart[HoldForm[x + 2 + 4 + x], First[pos] -> 4]

4 + 2 + 4 + x

Well here's a way. Find the position of the first occurrence of x:

expr = HoldForm[x + 2 + 4 + x];

pos = Position[expr, x, -1, 1];

Then:

ReplacePart[expr, pos -> 4]

4 + 2 + 4 + x

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created Sep 23, 2014 at 22:51

Well here's a way. First collect the positions of all x:

pos = Position[HoldForm[x + 2 + 4 + x], x];

Then:

ReplacePart[HoldForm[x + 2 + 4 + x], First[pos] -> 4]

4 + 2 + 4 + x